Optical fiber capped at end with diffractive film, and manufacturing method therefor

ABSTRACT

Affords efficiently and at low cost optical fibers capped at the end with a working, tiny optically diffractive film. An optical fiber includes a diffractive film formed onto an endface thereof, or onto the endface of a collimator joined to the endface of the fiber; the diffractive film includes a transparent DLC (diamond-like carbon) layer; and the DLC layer includes a modulated-refractive-index diffraction grating containing local regions of relatively high refractive index and local regions of relatively low refractive index.

TECHNICAL FIELD

The present invention relates optical fibers having an opticallydiffractive film on an end thereof, and to methods of manufacturing suchoptical fibers. More specifically, the invention relates to opticalfibers capped on an end with an optically diffractive film havingwavelength-division multiplexing/demultiplexing,power-splitting/combining, polarization-divisionmultiplexing/demultiplexing, wave-plate, or optical-isolatorfunctionality, and to methods of manufacturing such optical fibers.

BACKGROUND ART

As is widely known, diffractive optical elements that producediffraction of light can be employed in a variety of applications. Forexample, wavelength multiplexers/demultiplexers, optical couplers,optical isolators, and like devices used in optical communicationsfields can be manufactured employing diffractive optical elements.

Diffractive optical elements generally are manufactured by forming adiffraction-grating layer on a transparent substrate. Diffractiveoptical elements are grossly divided, based on structural differences inthe diffraction-grating layer, into modulated-refractive-index andsurface-relief types.

FIG. 14 depicts, in a schematic sectional view, an example of amodulated-refractive-index type of diffractive optical element. Itshould be understood that in the drawings for the present application,dimensional proportions such as width and thickness have been altered asappropriate in order to clarify and simplify the figures, and do notreflect the proportions in their actual relationships. Thismodulated-refractive-index optical element includes adiffraction-grating layer 12 a formed on a transparent substrate 11,wherein a modulated-refractive-index structure has been created in thediffraction-grating layer 12 a. In particular, local regions having arelatively small refractive index n₁ and local regions having arelatively large refractive index n₂ are periodically formed inalternation in the diffraction-grating layer 12 a. This enables theoccurrence of a diffraction phenomenon originating in the phasedifference that arises between light passing through the regions of lowrefractive index n₁ and light passing through the regions of highrefractive index n₂.

The diffraction-grating layer 12 a having the modulated-refractive-indexstructure can be formed utilizing for example a material whoserefractive index is increased by the material undergoing energy-beamirradiation. For instance, increasing the refractive index of Ge-dopedquartz glass by means of ultraviolet irradiation is known. Likewise,irradiating quartz glass with X-rays to increase the refractive index ofthe glass is known. Accordingly, a diffraction-grating layer 12 a asillustrated in FIG. 14 can be created by depositing a quartz-glass layerof refractive index n₁ onto a transparent substrate 11 and irradiatingthe glass layer with an energy beam in a periodic pattern to raise therefractive index locally to n₂.

FIG. 15 illustrates, in a schematic sectional view, an example of asurface-relief type of diffractive optical element. This surface-reliefdiffractive optical element includes a diffraction-grating layer 12 bformed on a transparent substrate 11, wherein a relief structure hasbeen embossed in the diffraction-grating layer 12 b. In particular,local regions having a relatively large thickness and local regionshaving a relatively small thickness are periodically formed inalternation in the diffraction-grating layer 12 b. This enables theoccurrence of a diffraction phenomenon originating in the phasedifference that arises between light passing through the regions oflarge thickness and light passing through the regions of smallthickness.

The diffraction-grating layer 12 b having the surface-relief structurecan be formed by for example depositing a quartz glass layer onto atransparent substrate 11 and employing photolithography and etching toprocess the glass layer.

FIG. 16 depicts, in a schematic sectional view, one more example of amodulated-refractive-index type of diffractive optical element. Themodulated-refractive-index optical element of FIG. 16 resembles that ofFIG. 14, but within a diffraction-grating layer 12 c in FIG. 16 localregions having refractive indices n₁, n₂, n₃ of three levels that differfrom each other are arrayed periodically. Local regions in this wayhaving three levels of refractive indices n₁, n₂, n₃ can be formedwithin a diffraction-grating layer 12 c by for example depositing onto asubstrate 11 a quartz glass layer of refractive index n₁ and irradiatingthe glass layer with an energy beam having two different energy levels.

By means of a diffraction grating that contains local regions whoserefractive indices are multilevel, diffraction efficiency can beimproved by comparison to the case with a diffraction grating thatcontains regions whose refractive indices are simply binary.“Diffraction efficiency” herein means the ratio of the sum of thediffracted light energies to the energy of the incident light. Thismeans that from the perspective of putting diffracted light to use,greater diffraction efficiency is to be preferred.

FIG. 17 represents, in a schematic sectional view, one more example of asurface-relief type of diffractive optical element. The surface-reliefoptical element of FIG. 17 resembles that of FIG. 15, but within adiffraction-grating layer 12 d in FIG. 17 local regions havingthicknesses in three levels that differ from each other are arrayedperiodically. Local regions in this way having refractive thicknesses inthree levels can be formed within a diffraction-grating layer 12 d byfor example depositing onto a substrate 11 a quartz glass layer andrepeating a photolithographic and etching process on the glass layer twotimes. Thus by means of a diffraction grating that contains localregions having a multilevel profile, diffraction efficiency can beimproved by comparison to the case with a diffraction grating thatcontains simple binary thicknesses.

It should be noted that while modulated-refractive-index diffractiongratings in which the refractive indices within the diffraction gratinglayer are varied in stages are illustrated in FIGS. 14 and 16, alsoformable are modulated-refractive-index diffraction gratings in whichthe refractive indices are varied continuously. In that case the energylevel of the energy beam used for irradiating in order to raise therefractive index should be varied continuously.

FIG. 18 schematically represents an example of the use of a diffractiveoptical element in an optical communications application. In the figure,collimators C0, C1 and C2 are respectively joined to the end faces ofoptical fibers F0, F1 and F2. Parallel-ray beam L, introduced throughoptical fiber F0 and output via collimator C0, can be split by adiffractive optical element DE into, for example, a beam of wavelengthλ₁ and a beam of wavelength λ₂. This is because the diffraction angle ofthe beam will differ depending on the wavelength λ.

Thus a beam having a wavelength of λ₁ can be input from collimator C1into optical fiber F1, while a beam having a wavelength of λ₂ can beinput from collimator C2 into optical fiber F2. In other words, thedemultiplexing functionality of the diffractive optical element DE isexploited in this case. Of course, conversely, a beam of wavelength λ₁output from optical fiber F1 via collimator C1, and a beam of wavelengthλ₂ output from optical fiber F2 via collimator C2 can be combinedthrough the diffractive optical element DE and input into optical fiberF0 via collimator C0. This means that the diffractive optical element DEcan demonstrate multiplexing/demultiplexing functionality. Thus adiffractive optical element of this sort having wavelength-divisionmultiplexing/demultiplexing functionality is able to perform a crucialrole in wavelength-division multiplexed (WDM) optical communications.

Although modulated-refractive-index diffractive optical elements such asdescribed above are manufacturable in principle, in practice producingmodulated-refractive-index diffractive optical elements is problematic.The reason is because the amount of refractive-index variationobtainable by irradiating for example quartz glass with an energy beamis at the very most 0.002 or so, which is prohibitive of creating aneffective diffraction-grating layer.

Consequently, the general practice at present is—as set forth forexample in Patent Reference 1, Japanese Unexamined Pat. App. Pub. No.S61-213802, and in Non-Patent Reference 1, Applied Optics, Vol. 41,2002, pp. 3558–3566—to employ surface-relief types as diffractiveoptical elements. Nevertheless, the photolithography and etchingnecessary for fabricating relief diffractive optical elements areconsiderably complex manufacturing processes requiring a fair amount oftime and trouble, besides which controlling the etching depth withprecision is no easy matter. What is more, a problem with surface-reliefdiffractive optical elements is that since microscopic corrugations areformed in the element face, dust and dirt are liable to adhere.

Meanwhile, in a drop optical circuit such as represented in FIG. 18, thediffractive optical element DE, some several mm across, must be alignedand fixed in place with respect to the semiconductor laser LD and theoptical fibers F0 through F2 atop a (non-illustrated) support base. Thismeans that in a conventional diffractive optical element, the opticalfibers are separate, individual optical components, which costs troublein handling and has been prohibitive of scaling down the optical path.

An object of the present invention, in view of the situation as in theforegoing with prior technology, is efficiently and at low cost to makeavailable optical fibers capped on an end with a working, tiny opticallydiffractive film.

Patent Reference 1

Japanese Unexamined Pat. App. Pub. No. S61-213802.

Non-Patent Reference 1

Applied Optics, Vol. 41, 2002, pp. 3558–3566.

DISCLOSURE OF INVENTION

According to the present invention, an optical fiber is characterized inincluding an optically diffractive film formed onto an endface of thefiber, or onto the endface of a collimator joined to the endface of thefiber; the diffractive film, in including a transparent DLC(diamond-like carbon) layer; and the DLC layer, in including adiffraction grating containing local regions of relatively highrefractive index and local regions of relatively low refractive index.

A diffractive film of this sort allows a single optical beam thatincludes a plurality of wavelengths to be split into a plurality ofbeams depending on the wavelength, and is capable of functioning as awavelength-division multiplexer/demultiplexer that can cause a pluralityof beams having different wavelengths to combine into a unitary opticalbeam.

The diffractive film as such also allows an optical beam of a singlewavelength to be split into a plurality of beams, and is capable offunctioning as a power splitter/combiner that can cause a plurality ofsingle-wavelength beams to combine into a unitary optical beam.

Moreover, a diffractive film of this sort is capable of functioning as apolarization-division multiplexer/demultiplexer that can cause TE wavesand TM waves contained in an optical beam of a single wavelength toseparate and to unite. The diffractive film as such is also capable offunctioning as a wave plate with respect to either TE waves or TM wavescontained in a single-wavelength optical beam.

Another possibility according to the present invention is creatingoptical-isolator functionality in the diffractive film by combiningtherein a first DLC layer containing a diffraction grating having theabove-described polarization-division demultiplexing functionality, witha second DLC layer containing a diffraction grating having wave-platefunctionality. If the thickness of a diffractive film thus havingoptical-isolator functionality is 20 μm or less, then the endfaces oftwo optical fibers can abut and be connected via the diffractive filmwithout a collimator or condenser being required. The reason why isbecause the optical divergence in a micro-distance of 20 μm or less isnegligible. Accordingly, the end of an optical fiber capped with a thindiffractive film of this sort advantageously may be retained in aconnector for abutting the fiber end against and connecting it to theendface of another optical fiber. Here, a transparent interlayer may beinserted in between the first DLC layer and the second DLC layer.

Furthermore, the diffraction grating that the diffractive film includescan be such that the grating is functional with respect to lightcontaining wavelengths within a range of from 0.8 μm to 2.0 μm.

In a method according to the present invention of manufacturing anoptical fiber as related above, the high refractive-index regionscontained in the diffraction grating(s) can be formed by irradiating theDLC layer(s) in a predetermined pattern with an energy beam to raise therefractive index of the layer(s).

In an implementation in which a transparent interlayer is inserted inbetween the first DLC layer and the second DLC layer to createoptical-isolator functionality, the first DLC layer can be depositedonto an endface of the fiber, or onto the endface of a collimator joinedto the endface of the fiber and irradiated with an energy beam to formin a first predetermined pattern the high-refractive index regions inthe first DLC layer; the transparent interlayer and the second DLC layercan be deposited in turn; and the second DLC layer can be irradiatedwith an energy beam to form in a second predetermined pattern thehigh-refractive index regions therein; wherein when the second DLC layeris being irradiated in the second predetermined pattern with an energybeam, the transparent interlayer can act to prevent the energy beam fromhaving an effect on the first DLC layer.

The energy beam for raising refractive index can be chosen from an X-raybeam, an electron beam, or an ion beam, and the DLC layer(s) can bedeposited by a plasma CVD technique.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view representing in the present invention anoptical fiber having a diffractive film on one end thereof

FIG. 2 is a schematic sectional view depicting an example of a stage ina diffractive-film production technique in the present invention.

FIG. 3 is a schematic sectional view depicting an example of a stage ina diffractive-film production technique in the present invention.

FIG. 4 is a schematic sectional view depicting an example of a stage ina diffractive-film production technique in the present invention.

FIG. 5 is a schematic sectional view depicting thewavelength-demultiplexing action of a wavelength-divisionmultiplexer/demultiplexer in the present invention.

FIG. 6 is a graph showing one example of the relationship betweenwavelength and intensity distribution of light demultiplexed by awavelength-division multiplexer/demultiplexer in the present invention.

FIG. 7 is a schematic plan view showing an example of adiffraction-grating pattern in an optical power splitter in the presentinvention.

FIG. 8 is a schematic sectional view depicting the power-splittingaction in an optical power splitter in the present invention.

FIG. 9 is a planar view showing the beam distribution within a planeintersecting a plurality of diffraction beams from power-splitting bythe optical power splitter of FIG. 7.

FIG. 10 is a schematic sectional view depictingpolarization-demultiplexing action in a polarization demultiplexer inthe present invention.

FIG. 11 is a schematic axonometric drawing depicting the functioning inthe present invention of a diffractive film capable of operating as anoptical isolator.

FIG. 12 is a schematic view depicting an optical fiber having on an endthereof an optical isolator according to the present invention.

FIG. 13 is a schematic sectional view representing a situation in whichan optical fiber having according to the present invention an opticalisolator on an end thereof is connected by means of a fiber connector toanother optical fiber.

FIG. 14 is a schematic sectional view representing an example of aconventional modulated-refractive-index type of diffractive opticalelement.

FIG. 15 is a schematic sectional view illustrating an example of aconventional surface-relief type of diffractive optical element.

FIG. 16 is a schematic sectional view representing one more example of aconventional modulated-refractive-index type of diffractive opticalelement.

FIG. 17 is a schematic sectional view illustrating one more example of aconventional surface-relief type of diffractive optical element.

FIG. 18 is a schematic view representing a conventionalwavelength-demultiplexing circuit for optical communications.

FIG. 19 is a schematic view representing a conventional isolator used inoptical communications.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 schematically depicts one example of an optical fiber accordingto the present invention. In this example, a circularly cylindricalcollimator C is joined onto the endface of an optical fiber F thatincludes a core approximately 10 μm in diameter and has an outerdiameter of approximately 125 μm (including its cladding). Thecylindrical collimator C has a cross-sectional diameter of approximately3 mm, with light from the optical fiber F being output as a parallel-raybeam of approximately 350 μm in cross-sectional diameter.

Both the endfaces of the cylindrical collimator C are flat. Adistributed-refractive-index lens that functions as a lens by virtue ofthe refractive index varying in radial a continuum can be employed as acylindrical collimator C of this sort. Such distributed-refractive-indexlenses are manufactured as components made of glass or plastic usingion-exchange or dispersion polymerization techniques.

Of the two endfaces of the cylindrical collimator C, onto that endfaceto which the optical fiber F is not joined, an optically diffractivefilm DF containing a transparent DLC (diamond-like carbon) layer isformed. The DLC layer contains a modulated-refractive-index diffractiongrating formed by irradiating the layer with an energy beam such as anion beam to raise the refractive index locally. Accordingly, the opticalbeam output from the optical fiber F via the collimator Cis diffractedby the diffractive film DF and separated into a plurality of beams—forexample, L1, L2, L3, etc.—depending on the diffraction angle. It will bereadily appreciated that on the other hand, if this plurality of beamsL1, L2, L3, etc. is shone into the diffractive film DF in the reversedirection, the beams will be united by the diffractive film DF and inputinto the optical fiber F via the collimator C

Embodiment 1

FIGS. 2 through 4 are schematic sectional views that depict one exampleof a method of manufacturing a modulated-refractive-index diffractivefilm in Embodiment 1 of the present invention.

Onto an SiO₂ substrate 1, as represented in FIG. 2, having a 1.44refractive index and having a 5 mm×5 mm principal face, a DLC layer 2was deposited by plasma CVD to a thickness of 2 μm.

A gold mask 3 in FIG. 3 was formed onto the DLC layer 2 by a lift-offtechnique. In the gold mask 3, gold stripes of 0.5 μm width and 5 mmlength were arrayed in iterations at a pitch of 0.5 μm. That is, thegold mask 3 had a “line & space” pattern. The DLC layer 2 was thereafterimplanted in an orthogonal direction through the apertures in the goldmask 3 with an He ion beam 4 at a dose of 5×10¹⁷/cm² under an 800-keVacceleration voltage.

As a result, the regions within the DLC layer that were not implantedwith He ions had a refractive index of 1.55, while the refractive indexof the regions 2 a that were implanted with He ions was raised to 2.05.Such refractive-index variation in a DLC layer was far larger bycomparison to refractive-index variation that can be produced in quartzglass, enabling a diffraction-grating layer of sufficiently largediffraction efficiency to be created.

In FIG. 4, etching removal of the gold mask 3 has yielded amodulated-refractive-index diffractive film DF. It will be appreciatedthat the diffraction-grating layer 2 in this diffractive film DFcontains regions of two types—refractive index 1.55 and 2.05—and thus isa so-called binary-level diffraction-grating layer.

FIG. 5 depicts in a schematic sectional view wavelength-demultiplexingaction in an implementation in which the obtainedmodulated-refractive-index diffractive film DF was utilized as awavelength-division multiplexer/demultiplexer. In this sectionaldrawing, the section areas in black represent regions of relatively highrefractive index, while the section areas in white represent regions ofrelatively low refractive index. As is indicated in FIG. 5, if a singleoptical beam containing a plurality of wavelengths λ₁, λ₂, λ₃, λ₄, forexample, is made incident onto the diffractive film DF, the diffractionangles of the rays that pass through the diffractive film differ fromone another depending on the wavelength. This means that a singleincident beam containing a plurality of wavelengths can be separatedinto a plurality of diffraction beams that wavelength by wavelengthdiffer in advancing direction.

Of course, it should be evident that if the sense of the incident lightbeam and the diffraction beams indicated by the arrows in FIG. 5 isreversed, the diffractive film DF in FIG. 5 can be utilized as amultiplexer. It is to be noted that in implementations in which thediffractive film is utilized as a wavelength division demultiplexer, theoptical beam is generally made incident on the diffractive film at asuitable angle within a range of 0–70 degrees or so relative to a linenormal to the film surface. This angle range, however, relates to thesituation in which the high refractive-index regions have been formed inan orientation orthogonal to the face of the DLC layer; in a case inwhich, with the ion beam being diagonally directed onto the DLC layerface, the high refractive-index regions have been formed in the surfaceof the DLC layer at a slant for example, the incident angle of theoptical beam would be adjusted taking the angle of slant intoconsideration.

In the present Embodiment 1, a diffractive film DF having a line & spacepattern-in which by diagonal irradiation with an ion beam, gold stripesof 0.5 μm width were arrayed in iterations at a pitch of 0.5 μm—wasformed onto one of the endfaces of a collimator C and, as represented inFIG. 1, the other end of the collimator C was connected to the endfaceof an optical fiber F

FIG. 6 is a graph that schematically represents one example ofwavelength-demultiplexing results in the optical fiber F furnished atone end with the collimator C on the endface of which the diffractivefilm DF having the line & space pattern just noted was formed. Thehorizontal axis in the graph represents diffraction-light wavelength(nm), while the vertical axis represents the diffraction-light intensityin arbitrary units. In this case, light having a wavelength range of 1.5to 1.6 μm and a beam diameter of 350 μm was presented to the diffractivefilm DF via the optical fiber F and the collimator C (cf. FIG. 1). As aresult, five diffraction beams having wavelengths distributed at 20-nmintervals in a spectrum from 1.5 μm to 1.6 μm as shown in FIG. 6 wereproduced, with the five diffraction beams having approximately equalintensity. And with the diffraction efficiency in that case then beingnearly 99%, quite outstanding wavelength demultiplexing properties wereachieved.

It should be understood that in Embodiment 1, because a linear,one-dimensional diffraction-grating pattern is utilized the pluraldiffraction beams are present lying in a single plane that contains theincident beam. Nevertheless, it will be understood that by utilizing atwo-dimensional diffraction-grating pattern—as in Embodiment 2, whichwill be set forth next—in an orthogonal sectional plane the pluraldiffraction beams can be distributed two-dimensionally.

Embodiment 2

FIG. 7 shows, in a schematic plan view, a two-dimensionaldiffraction-grating pattern in a diffractive film in Embodiment 2. Thediffractive film in Embodiment 2 can be fabricated by the samemanufacturing process as was the case with Embodiment 1. In particular,the black regions in FIG. 7 represent where within the DLC layer therefractive index was raised by irradiating the regions with the He ionbeam, while the white areas represent regions that were not irradiatedwith the He ion beam. The black pattern was formed by combining 4 μm×4μm microcells, and therein had a periodicity of 132 μm. This means thatthe minimum linewidth in the diffraction-grating pattern illustrated inFIG. 7 is 4 μm.

FIG. 8 depicts, in a schematic sectional view, power-splitting action ina case in which the modulated-refractive-index diffractive film inEmbodiment 2 is employed as an optical coupler (power splitting device).In particular, if a beam of light of a single wavelength is madeincident on the diffractive film DF, the diffraction angle of the raysthat pass through the diffractive film differ from one another dependingon the order of diffraction. Consequently, an incident beam of a singlewavelength can be separated into a plurality of optical diffractionbeams.

FIG. 9 is a plan view representing beam distribution within a planeorthogonal to a diffraction beam having been power-split, as in FIG. 8,into a plurality by the optical coupler of FIG. 7. More specifically, anincident beam having power P can be split into 16 diffraction beams eachhaving power P/16.

In embodiment 2, an optical fiber F was connected to one endface of acollimator C onto the other endface of which a diffractive film DFhaving a diffraction-grating pattern as is illustrated in FIG. 7 wasformed, and light of 350 μm beam diameter having a wavelength of 1.55 μmwas made perpendicularly incident on the diffractive film DF (cf. FIG.1), wherein diffraction beams in 16 splits distributed in four-foldsymmetry as is represented in FIG. 9 were produced.

It will be appreciated that a diffraction-grating pattern of FIG. 7enabling the realization of a plural-diffraction-beam distributionpattern as is shown in FIG. 9 can, as is widely known, be found using aFourier transform.

Embodiment 3

In Embodiment 3, an optical fiber including a diffractive film havingpolarization-division multiplexing/demultiplexing functionality wasfabricated. In Embodiment 3 as well, a DLC diffraction-grating layerhaving a line & space pattern was formed by the same manufacturingprocess as was the case with Embodiment 1. In Embodiment 3, however,high refractive-index regions of 0.4 μm width were arrayed in iterationsat a pitch of 0.4 μm.

FIG. 10 depicts in a schematic sectional viewpolarization-demultiplexing action in an implementation in which themodulated-refractive-index diffractive film DF in Embodiment 3 isemployed as a polarization-division multiplexer/demultiplexer. Inparticular, if a TEM wave including a TE component and a TM component ismade incident on the diffractive film DF in Embodiment 3, the TE waveand the TM wave will, depending on the difference in polarizationbetween them, be diffracted at diffraction angles that differ from eachother. For example, as is illustrated in FIG. 10, the TE wave isobtained as a 0 order diffraction beam, while the TM wave is obtained asa −1 order diffraction beam. The splitting off of TE and TM waves is inthis way made possible.

In Embodiment 3, an optical fiber F (cf FIG. 1) was connected to oneendface of a collimator C onto the other endface of which a diffractivefilm DF having a diffraction-grating pattern—in which by diagonalirradiation with an ion beam, high refractive-index regions of 0.4 μmwidth were arrayed in iterations at a pitch of 0.4 μm—was formed, andlight of 350 μm beam diameter having a wavelength of 1.55 μm was outputvia the diffractive film DF, wherein a TE-polarized wave and aTM-polarized wave could be split off.

Embodiment 4

Prepared in Embodiment 4 was an optical fiber F(cf. FIG. 1), furnishedat an end thereof with a collimator C on an endface of which was formeda diffractive film DF having wave-plate functionality. With thediffractive film of Embodiment 4 as well, a DLC diffraction-gratinglayer having a line & space pattern was formed by the same manufacturingprocess as was the case with Embodiment 1. In Embodiment 4, however,high refractive-index regions of 0.2 μm width were arrayed in iterationsat a pitch of 0.2 μm. A further difference was that the optical fiber Futilized in Embodiment 4 was not a single-mode fiber as was the casewith the other embodiments, but was a polarization-holding fiber.

Linearly polarized light 1.55 μm in wavelength was input into what wasthus a polarization-holding fiber F, and the beam output from thediffractive film DF via the collimator C was converted into circularlypolarized light. This means that the diffractive film DF in Embodiment 4functioned as a quarter-wave plate, producing a polarization conversioneffect.

Embodiment 5

FIG. 11, a schematic axonometric drawing, models the functioning as anoptical isolator of a diffractive optical element, in Embodiment 5, thatis practicable for an optical fiber. In this diffractive optical elementa first DLC layer 32 has been formed onto a first principal face of aquartz glass substrate 31, and a second DLC layer 33 has been formedonto the second principal face. Then the same diffraction grating as inEmbodiment 3 has been formed in the first DLC layer 32, and the samediffraction grating as in Embodiment 4 has been formed in the second DLClayer 33.

If an optical beam of 1.55 μm wavelength is made incident on thediffractive optical element of FIG. 11, though a ray 35 having passedthrough the first diffraction-grating layer 32, functioning as apolarization demultiplexer, and the second, diffraction optics layer 33,functioning as a quarter-wave plate, is back-reflected from some object,it cannot pass through the quarter-wave plate 33 and the polarizationdemultiplexer 32—which operate interactively as an optical isolator—andreturn.

As represented schematically in FIG. 12, in Embodiment 5 a diffractivefilm DF having optical-isolator functionality was formed onto oneendface of a collimator C0, and an optical fiber F0 was joined to theother endface of the collimator. With this diffractive film DC, a firstDLC layer D1 was formed onto the one endface of the collimator C0, andthe same diffraction grating as in Embodiment 3 havingpolarization-demultiplexing functionality was formed in the first DLClayer D1. After that, an SiO₂ interlayer M and a second DLC layer D2were in turn formed onto the first DLC layer D1. It will be appreciatedthat the SiO₂ interlayer and the second DLC layer D2 can be formed by aCVD technique or an EB (electron beam) vapor deposition technique. Thenthe same diffraction grating as in Embodiment 4 having quarter-waveplatefunctionality was formed in the second DLC layer D2. In doing so, whilethe second DLC layer D2 was being irradiated with the ion beam theinterlayer M dependably prevented the ion beam from entering the firstDLC layer D1.

From the optical fiber F0 via the collimator C0 optical beam L0, asrepresented in FIG. 12, of 1.55 μm wavelength and 350 μm cross-sectionaldiameter was output from the diffractive film DF. In thatimplementation, even if beam L0 having passed through the firstdiffraction-grating layer D1, acting as a polarization demultiplexer,and the second diffraction-grating layer D2, acting as a quarter-waveplate, after being presented to another optical fiber F1 via anothercollimator C1 is back-reflected at some interface and comes back as beamLb, the light would not be able to pass through the quarter-wave plateD2 and the polarization demultiplexer D1, operating interactively as anoptical isolator, and return into the optical fiber F0. As theextinction ratio in that instance—being the ratio of the incidentluminous intensity to the transmitted luminous intensity of lightreturning to the first diffraction-grating layer D1—a value of over 40dB was obtained.

Embodiment 6

FIG. 13 represents in schematic cross-section a situation in which anoptical fiber according to Embodiment 6 is joined to another opticalfiber via an optical connector. In this Embodiment 6, a diffractive filmDF having the same optical-isolator functionality as was the case withEmbodiment 5 has been formed onto, directly in contact with, the endfaceof an optical fiber F0. In this case, the diffractive film DF has awhole thickness of 20 μm or less.

The end of the optical fiber F0 where it is capped with the diffractivefilm DF is retained by an FC-type optical connector FC0. The opticalconnector FC0 includes a plug portion PL0 that retains the end portionof the fiber, and a cinch nut N0 for joining the plug portion to anadapter AD0. In a like manner, the end portion of the other opticalfiber F1 is retained by means of an FC-type optical connector FC1. Theoptical connector FC1 too includes a plug portion PL1 that retains theend portion of the fiber, and a cinch nut N1 for joining the plugportion to an adapter AD1. The two adapters AD0 and AD1 are joined toeach other by means of tiny bolts and nuts (not illustrated).

As illustrated in FIG. 13, in Embodiment 6, via the diffractive film DFhaving optical-isolator functionality the two optical fibers F0 and F1can be directly connected without requiring collimating or condensinglenses as would otherwise be the case with conventional opticalisolators. This is because the diffractive film DF havingoptical-isolator functionality can be formed at a whole width of 20 μmor less and therefore, influence of divergence on the optical beam canat this micro-distance of 20 μm or less be averted. Here, in order toabut the common ends of the two optical fibers F0 and F1 against eachother, a (not illustrated) spring is built into the connectors FC0 andFC1.

For reference, a conventional isolator used in optical communications isschematically represented in FIG. 19. In this figure, an optical beamoutput from the optical fiber F0 via the collimator C0, after passing afirst polarizer P1, a Faraday rotator FR, and a second polarizer P2,which are included in an isolator IL, is introduced into the opticalfiber F1 via the collimator C1.

As is clear from a comparison of FIGS. 13 and 19, in Embodiment 6, twooptical fibers can abut and be connected via an optical isolator in anextraordinarily compact fashion relative to what as been the case todate. It will be readily apparent that although in FIG. 13 an FC-typeconnector is employed in order to abut and connect the two opticalfibers via the optical isolator, connectors in a variety of other formsmay be employed. Furthermore, the two optical fibers may be brought intoabutment and connected via the optical isolator within a V-shaped grooveformed on the face of a silicon substrate.

While in the foregoing embodiments examples in which He ion irradiationwas employed to raise the refractive index of the DLC layers wereexplained, it will be appreciated that ion irradiation of other kinds,X-ray irradiation, or electron-beam irradiation for example can also beemployed in order to raise the refractive index of the DLC layers.Likewise, in the above-described embodiments explanation regardingincident light within a range of wavelengths from 1.5 μm to 1.6 μm wasmade, but in the present invention optical fibers having a diffractivefilm usable for light having any wavelengths within a 0.8 μm to 2.0 μmband with the potential of being employed in optical communicationsapplications can be manufactured.

Furthermore, although in the above-described embodiments, optical fibershaving a diffractive film containing a binary-level diffraction gratinglayer were explained, it will be readily appreciated that diffractiongratings with multilevel or continuously modulated refractive indexprofiles can also be formed within the DLC layers. In such cases, theDLC layers may be irradiated with, for example, an energy beam in whichthe energy level and/or dose is varied.

Still further, in the embodiments described above, in most cases thediffractive film DF is formed onto the endface of a collimator, but itwill be readily understood that in implementations in which it is sodesired, a diffractive film DF may as in Embodiment 6 be formed directlyonto the enface of an optical fiber.

INDUSTRIAL APPLICABILITY

As given in the foregoing, the present invention enables practicaloptical fibers capped at the end with a modulated-refractive-indexdiffractive film to be made available efficiently and at low-cost.Moreover, in modulated-refractive-index diffractive films maderealizable by utilizing DLC layers, since microscopic corrugations arenot present in the surface as with surface-relief diffractive opticalelements, the surface is unlikely to be contaminated, and even if it iscontaminated can be readily cleansed. What is more, because its DLClayer has high resistance to wear, a diffractive film of the presentinvention formed on the end portion of an optical fiber is advantageousfrom the perspective that the film surface is unsusceptible to damage.

1. An optical fiber comprising: an optically diffractive film formed onan end portion of the optical fiber; a transparent DLC material includedin said diffractive film, said transparent DLC material being formedeither onto the face of the optical fiber end portion, or onto anendface of a collimator joined to the endface of the optical fiber; anda diffraction grating included in said DLC material, said diffractiongrating containing local regions of relatively high refractive index andlocal regions of relatively low refractive index.
 2. An optical fiber asset forth in claim 1, wherein said diffractive film allows a singleoptical beam including a plurality of wavelengths to be split into aplurality of beams depending on the wavelength, and functions as awavelength-division multiplexer/demultiplexer for causing a plurality ofbeams having different wavelengths to combine into a unitary opticalbeam.
 3. An optical fiber as set forth in claim 1, wherein saiddiffractive film allows a single-wavelength optical beam to be splitinto a plurality of beams, and functions as a power splitter/combinerfor causing a plurality of single-wavelength beams to combine into aunitary optical beam.
 4. An optical fiber as set forth in claim 1,wherein said diffractive film has polarization-divisionmultiplexer/demultiplexer functionality for separating, and causing tounite, TE waves and TM waves contained in a single-wavelength opticalbeam.
 5. An optical fiber as set forth in claim 1, wherein saiddiffractive film has wave-plate functionality with respect to either TEwaves or TM waves contained in a single-wavelength optical beam.
 6. Amethod of manufacturing an optical fiber as set forth in claim 1, theoptical-fiber manufacturing method comprising a step of forming the highrefractive-index regions contained in the diffraction grating(s) byirradiating said DLC material in a predetermined pattern with an energybeam to raise the refractive index of the DLC material.
 7. Anoptical-fiber manufacturing method as set forth in claim 6, wherein theenergy beam is selected from an X-ray beam, an electron beam, or an ionbeam.
 8. An optical-fiber manufacturing method as set forth in claim 6,wherein said DLC material is deposited by a plasma CVD technique.
 9. Anoptical fiber comprising: an optically diffractive film formed on an endportion of the optical fiber; a first transparent DLC layer and a secondtransparent DLC layer included in said diffractive film and laminated inturn onto an face of the optical fiber end portion, or onto an endfaceof a collimator joined to the endface of the optical fiber; a firstdiffraction grating included in said first DLC layer, said firstdiffraction grating containing local regions of relatively highrefractive index and local regions of relatively low refractive index; asecond diffraction grating included in said second DLC layer, saidsecond diffraction grating containing local regions of relatively highrefractive index and local regions of relatively low refractive index;wherein said first DLC layer has polarization-division demultiplexingfunctionality for splitting by polarization TE waves and TM wavescontained in a single-wavelength optical beam, said second DLC layer haswave-plate functionality with respect to either TE waves or TM wavescontained in a single-wavelength optical beam, and said first and secondDLC layers function interactively as an optical isolator.
 10. An opticalfiber as set forth in claim 9, wherein said diffractive film is formedonto the endface of the optical fiber, and has a thickness of 20 μm orless.
 11. An optical fiber as set forth in claim 10, further comprisinga connector for retaining the optical fiber end portion where thediffractive film is formed and for abutting the fiber end portionagainst and connecting it to an endface of another optical fiber.
 12. Anoptical fiber as set forth in claim 9, further comprising a transparentinterlayer inserted in between said first DLC layer and said second DLClayer.
 13. A method of manufacturing an optical fiber as set forth inclaim 9, the optical-fiber manufacturing method comprising a step offorming the high refractive-index regions contained in the diffractiongrating(s) by irradiating said DLC layer(s) in a predetermined patternwith an energy beam to raise the refractive index of the layer(s). 14.An optical-fiber manufacturing method as set forth in claim 13, whereinthe energy beam is selected from an X-ray beam, an electron beam, or anion beam.
 15. An optical-fiber manufacturing method as set forth inclaim 13, wherein said DLC layer(s) is deposited by a plasma CVDtechnique.
 16. An optical fiber as set forth in claim 1 or 9, whereinsaid diffractive film includes the diffraction grating being functionalwith respect to light containing wavelengths within a range of from 0.8μm to 2.0 μm.
 17. A method of manufacturing the optical fiber set forthin claim 12, the optical-fiber manufacturing method comprising steps of:depositing said first DLC layer onto the endface of the optical fiber,or onto the endface of the collimator joined to the endface of theoptical fiber; forming said high-refractive index regions in the firstDLC layer by irradiating it with an energy beam to raise its refractiveindex in a first predetermined pattern; depositing said transparentinterlayer and said second DLC layer in turn; and forming saidhigh-refractive index regions in said second DLC layer by irradiating itwith an energy beam to raise its refractive index in a secondpredetermined pattern; wherein when said second DLC layer is beingirradiated in said second predetermined pattern with an energy beam,said transparent interlayer acts to prevent the energy beam from havingan effect on said first DLC layer.
 18. An optical-fiber manufacturingmethod as set forth in claim 17, wherein the energy beam is selectedfrom an X-ray beam, an electron beam, or an ion beam.
 19. Anoptical-fiber manufacturing method as set forth in claim 17, whereinsaid first and second DLC layers are deposited by a plasma CVDtechnique.